Preparation of synthetic steamcracker feed from cycloalkanes (or aromatics)
on zeolite catalysts
Jens Weitkamp,*a Andreas Raichle,a Yvonne Traa,a Martin Ruppb and Franz Fuderb
a Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany.
E-mail: jens.weitkamp@po.uni-stuttgart.de
b VEBA OEL AG, D-45876 Gelsenkirchen, Germany
Received (in Cambridge, UK) 23rd December 1999, Accepted 4th February 2000
Methylcyclohexane is converted into a high-quality steam-
cracker feed over acidic zeolites with appropriate pore
systems, thereby opening a new route for the utilisation of
surplus aromatics.
Steamcrackers1 are widely employed for the production of
ethylene and propylene from naphtha,2 ethane or similar light
hydrocarbons. One of the by-products of steamcracking is
pyrolysis gasoline which is rich in aromatics. Given the
forecasted growth in the worldwide demand of ethylene3 and
propylene4 from, respectively, 57 3 106 and 30 3 106 tonne
yr21 in 1990 to ca. 140 3 106 and 75 3 106 tonne yr21 in 2010,
the production of pyrolysis gasoline will necessarily increase as
well. Up till now, pyrolysis gasoline has been used as an
aromatics-rich and, hence, high-octane petrol component. With
the so-called Auto Oil Programme of the European Community,
the aromatics content of petrol has to be diminished from 45 to
Fig. 1 Conversion of methylcyclohexane on H-ZSM-5 at various tem-
peratures.
< 42 vol% until the year 2000, and to < 35 vol% until 2005.5
Therefore, new outlets for surplus aromatics are urgently
needed. We report here on a novel catalytic chemistry by which
cycloalkanes (or aromatics, since these can easily be hydro-
genated into cycloalkanes by state-of-the-art processes) are
converted predominantly into light linear alkanes (ethane,
propane, n-butane and n-pentane) the mixture of which is a
premium steamcracker feedstock for high ethylene and propyl-
ene yields.6
Previous work on ring opening of cycloalkanes mostly relies
on hydrogenolysis over noble metal catalysts designed predom-
inantly to yield branched alkanes.7–9 Such hydrocarbons are,
however, less suitable as a steamcracker feed, since large
amounts of undesired methane will be formed. Only few reports
deal with cracking of cyclic hydrocarbons over acidic zeolites in
an excess of hydrogen,10–12 and yields of n-alkanes during the
conversion of cycloalkanes or aromatics with seven or more
carbon atoms are reported to be low.
Y
isobutane (Table 1, entry 9 and Fig. 3), which is considered as a
quantitative measure for the relative contributions of mono-
molecular Haag–Dessau cracking17 versus conventional bimo-
lecular cracking. In the narrower pores of zeolite ZSM-5 and
with increasing temperature, Haag–Dessau cracking, which
produces large amounts of ethane and propane, is favoured over
the spatially more demanding bimolecular mechanism which
leads predominantly to isobutane. In line with this inter-
pretation, much more hydrogen is incorporated into the cracked
products on H-ZSM-5 than on H–Y (Table 1, entry 10): Clearly,
H2 can be activated on acidic zeolites, as demonstrated
previously.12,18,19
In conclusion, methylcyclohexane can be converted over
acidic zeolites with suitable pore systems into a high-quality
synthetic steamcracker feed consisting predominantly of eth-
On zeolite H-ZSM-5 at 400 °C, methylcyclohexane is
converted13 with a yield of 70.6% into ethane (9.4%), propane
(47.7%), n-butane (12.4%) and n-pentane (1.1%), see Fig. 1 and
Table 1, whereas on zeolite H-Y, much lower yields (42.8%) of
these desirable products are obtained. On H-ZSM-5, the most
important by-products are branched alkanes, viz. 17.6% iso-
butane, 6.2% isopentane and 0.6% isohexanes. Whereas on
H-ZSM-5 virtually no aromatics are found, the total yield of
aromatics on H-Y amounts to 15.9% (benzene 2.2%, toluene
6.4%, C8-aromatics 5.6% and C9-aromatics 1.7%). We tenta-
tively attribute this difference to the lower activity of the larger-
pore zeolite for cleavage of endocyclic carbon–carbon bonds.14
The difference in selectivities becomes even more pronounced
at lower conversions (Fig. 2) or at high times-on-stream (Table
1, entry 7). The deactivation observed on zeolite H-Y correlates
qualitatively with the mass ratio of carbon in the coke formed on
the catalyst15 and in the methylcyclohexane fed (Table 1, entry
8).
Table 1 Conversion of methylcyclohexane at 400 °C and 30 min TOSa
Entry
Catalyst
H-ZSM-5
H-Y
1
2
3
4
5
XM-CHx (%)
99.9
4.0
70.6
24.4
0.2
99.5
0.6
42.8
39.3
0.8
Ymethane (%)
Yn-alkanesb (%)
Yi-alkanes (%)
Ycycloalkanes (%)
6
7
8
9
Y
aromatics (%)
0.7
15.9
0.84
1.76
0.15
1.2
Yn-alk.b(440 min TOS)/Yn-alk. (30 min TOS) 1.00
m
C, coke/mC,M–CHxc (%)
< 0.05
0.76
2.3
CMRd
10
H2 incorporatione
a Time-on-stream.b Yield of n-alkanes = Yethane + Ypropane + Yn-butane
+
Yn-pentane, the individual yields being defined in the usual manner, e.g.
Yethane = (2/7) 3 (n˙ethane, out/n˙M–CHx, in).c Mass ratio of carbon in the coke
formed on the catalyst after 500 min time-on-stream and in the
methylcyclohexane cumulatively fed within the same time.d Cracking
The pronounced differences in the catalytic behaviour of both
zeolites can be accounted for by introducing the so-called
mechanism ratio, see text.e n˙H , consumed/n˙M–CHx, fed
.
2
cracking mechanism ratio,16 CMR · (Ymethane + YC hydrocarbons)/
2
DOI: 10.1039/a910284l
Chem. Commun., 2000, 403–404
This journal is © The Royal Society of Chemistry 2000
403
Weitkamp, Jens
